The invention relates to a lower thread tension adjusting device of a sewing machine having a loop taker which is rotated in a horizontal plane to catch a thread loop formed at the needle as the latter goes up from the lower dead point thereof during a fabric stitching operation. The device of the invention is so designed as to enable the sewing machine operator to adjust the lower thread tension from outside of the sewing machine in reference to indices even while driving the sewing machine.
When a fabric is sewn with a sewing machine, it is generally required to determine an upper thread tension in dependence upon a kind or thickness of the fabric to be sewn, and accordingly it becomes necessary to adjust a lower thread tension to be balanced with the determined upper thread tension. Hitherto, the lower thread tension has been adjusted by manipulation of an adjusting screw, after once interrupting the stitching operation, removing a needle plate and then taking out a bobbin carrier from a loop taker. More particularly, in reference to FIG. 1, the conventional lower thread adjusting device 4 is composed of an arcuate base plate 1 secured to an inner wall of a bobbin carrier 5 which carries a bobbin 7 loaded with a lower thread 8, a resilient plate 2 secured at one end thereof to the arcuate plate 1, and an adjusting screw 3 passed through the resilient plate 2 and screwed into the arcuate base plate 1. The bobbin carrier 5 is received in a loop taker 6 and is kept stationary while the loop taker 6 is rotated on a vertical shaft 6a. As shown, the arcuate base plate 1 is formed with an inclined slit 1a and has a cutout 1b formed on the upper edge thereof on the left side of the inclined slit 1a. The lower thread 8 supplied from the bobbin 7 is inserted down into the inclined slit (1a) and guided up between the arcuate base plate 1 and the resilient plate 2 and is passed through the cutout 1b up to the needle plate (not shown). Thus, the lower thread 8 is pressed between the arcuate base plate 1 and the resilient plate 2 and has a tension due to the frictional resistance. In order to vary the lower thread tension, the adjusting screw 3 is fastened or loosened to vary the pressure of the resilient plate 2 applied to the lower thread 8. For manipulation of the adjusting screw 3, it is unavoidable according to the conventional device, to once interrupt the stitching operation of the sewing machine and to take the bobbin carrier 5 out of the loop taker 6. Moreover it is very difficult and time consuming to properly determine the lower thread tension because the tension adjustment has to be carried out by a guess-work, which often requires the readjustments of the adjusting screw 3.